Once you've picked your mushrooms from the wild you will need to try and act quickly to get them prepared and ready to be eaten. Once a mushroom has been cut at the stem it will be ready to die and will only keep for a couple of days if kept in a good condition.
Mushrooms when picked should be placed in a wooden basket to enable them to breathe properly. When taken home you will need to check them for maggots and other bugs, and then remove any excess dirt and grit which can be quite frustrating to eat.
A mushroom brush can be used quite easily just to brush away any visible dirt from the surface of the mushroom, either on its cap, in its gills (which can be quite fiddly) and on its stem. The brush can be damp if you like, and you could also use a piece of tissue or kitchen paper. You should never wash a mushroom - mushrooms are very good at absorbing moisture and by washing a mushroom you will be removing any of its flavour. Some people place shop brought field mushrooms in a bucket of water whilst they scrub them - they are simply ruining the flavour of an already very flavourless mushroom.

To check for maggots in your mushroom you can usually just slice through the stem in a horizontal motion. If very small holes are visible then this is likely to be the entrance point of the maggot into the mushroom (tunnelling up through the ground and up the stem). Sometimes you can slice the mushroom and cut away any parts that the maggot has encountered, but often you will find where one maggot exists there will be more, and this can be quite frustrating. It is best to check for maggots in this way as soon as you have picked the mushroom in the wild; this way you can simply return it to the woodland floor where the maggots can continue to eat the mushroom and continue its natural lifecycle and so it can drop any more of its spores. You may find that some maggots or bugs enter the mushroom from its cap, so when you are slicing your mushrooms through the cap be aware that any visible holes again could have been caused by maggots. Although any maggots or bugs that get consumed along with the mushroom are unlikely to do you any harm, its probably best to avoid them.
Once the mushrooms have been wiped clean using a tissue or brush and have been checked for maggots they will be ready to eat. You can choose to grill, fry or bake your mushrooms as you please. You might even want to dry or freeze your mushrooms, to enable you to enjoy them at a later date. This can be a good idea if you have picked more mushrooms then you require for that day, to save them from going to waste.